1. Field of the Invention
The present invention relates to a rapid thermal annealing (RTA) semiconductor manufacturing system and method. More specifically, the present invention relates to an RTA system and method having improved temperature sensing, monitoring and control.
2. Description of the Related Art
Rapid thermal annealing (RTA) is a semiconductor fabrication technique using short-time, high temperature processing to avoid unwanted dopant diffusion that would otherwise occur at the high processing temperatures of 900.degree. C. to 1000.degree. C. or greater that are used to dissolve extended defects in silicon (Si) and gallium arsenide (GaAs). The duration of an RTA process ranges from seconds to a few minutes so that semiconductor substrates are subjected to high temperatures only long enough to attain a desired process effect but not so long that a large degree of dopant diffusion takes place. RTA is typically performed in specially-designed systems rather than conventional furnaces or reactors which include susceptors, wafer boats and reactor walls having a large thermal mass which prevents performance of rapid thermal cycling. Early RTA processes used lasers as an energy source, allowing a high degree of heating to occur within fractions of a microsecond without significant thermal diffusion. Unfortunately, the wafer surfaces had to be scanned by small spot-size laser beams, causing lateral thermal gradients and wafer warping.
Subsequently, large-area incoherent energy sources were developed to overcome these limitations. These energy sources emit radiant light, which then heats the wafers, allowing very rapid and uniform heating and cooling. RTA systems have been developed in which wafers are thermally isolated so that radiant, not conductive, heating and cooling predominates. Temperature uniformity is a primary design consideration in these systems so that thermal gradients, which cause slip and warpage, are avoided. RTA systems use various heat sources including arc lamps, tungsten-halogen lamps, and resistively-heated slotted graphite sheets.
Several difficulties arise in achieving temperature uniformity. First, to raise the temperature of a semiconductor wafer of course requires heating of the slide carriers and insertion equipment for handling the wafer. The large thermal mass of slide carriers and insertion equipment extend the process times to at least fifteen to thirty minutes to obtain reproducible results. Significant changes in the doping profile of the wafer can occur over this time, causing difficulty in forming a desired structure in the substrate. For example, the precise alignment of shallow junctions becomes difficult to control when the temperature is not controllable.
A second problem is that dopants such as arsenic can be lost through preferential evaporation effects. In GaAs, arsenic loss is severe with considerable deterioration of the semiconductor material unless the semiconductor is appropriately capped.
Temperature uniformity is typically tested by measuring the emissivity of a semiconductor wafer. Emissivity is defined as the ratio of power per unit area radiated from a surface to the power radiated by a black body at the same temperature when radiation is produced by the thermal excitation or agitation of atoms or molecules. When a semiconductor wafer is heated, such as occurs in rapid thermal annealing, the temperature of the wafer is raised and the increase in temperature is detectable by an optical signal with a characteristic spectrum that is indicative of the wafer temperature. A measurement of emissivity quantifies the characteristic spectrum.
Referring to FIG. 1, an intensity-wavelength plot of the frequency spectrum response 100 of an infrared pyrometer is shown. In a typical conventional rapid thermal anneal system, a single fixed-wavelength pyrometer, for example having a wavelength of 2.7.mu., is used to measure temperature, typically at one or two positions. The frequency spectrum detected by the infrared pyrometer is neither narrow-band limited or broad-band limited, having a detection band of a few angstroms of receptive wavelength in the vicinity of the infrared region. One problem which arises using the infrared pyrometer to detect emissivity is that only wavelengths in the relatively limited range of the infrared spectrum are detected.
Thus, the conventional usage of an infrared pyrometer ignores emissivity in other regions of the spectrum, tantamount to an assumption that emissivity occurs at a constant level across a broad spectrum and that the infrared regions is highly representative of the emissivity of the broad spectrum. However, these assumptions are erroneous.
As a semiconductor wafer is illuminated, the wafer absorbs part of the energy and reflects part of the energy. The relative amount of energy reflected and absorbed is highly dependent on the type of films on the wafer, which may be highly variable from wafer to wafer. The relative amount of energy that is reflected and absorbed is highly position-dependent in the wafer. The wafer surface generally includes various oxides, polysilicon, deposited oxides and the like, generally having variable thicknesses and types. Differences in both the type of material and the thickness of the material on the semiconductor wafer relate to variability in the absorption and reflectivity of local areas of the wafer, causing variations in emissivity at different regions of the semiconductor wafer. For example, absorption of radiant heat by the semiconductor wafer is related to the free carrier concentration so that the heating rate for heavily doped material is more rapid than for semiconductor wafers with less doping.
Nulls occasionally occur in which substantially no energy is reflected and thereby detected by the infrared pyrometer. In particular, the various types of deposits and deposition thicknesses act as a quarter-wave plate in which energy is absorbed in a material of a particular type and thickness which is coincident with the effective wavelength of the pyrometer so that a quarter-wave path difference with a relative phase shift of 90.degree. occurs between ordinary and extraordinary waves. Thus, substantially all of the energy at the effective wavelength of the pyrometer is absorbed in the material and very little is reflected. The pyrometer badly misjudges the temperature of the wafer in these regions, measuring a temperature that is much lower than the actual temperature.
The temperature measurement system is typically used in a feedback control system which responds to the detected low temperature by increasing the intensity of the heating lamps or extending the duration of annealing. The increase in RTA processing damages or destroys the semiconductor wafer in process.
Present day rapid thermal anneal systems typically address the problems of Emissivity measurement variations and temperature measurement inaccuracies by attempting to construct an ideal RTA chamber, specifically an RTA chamber which is most equivalent to a black body radiator so that the only energy absorbing component in the chamber is the semiconductor wafer. However, even with an ideal RTA chamber, absorption by the semiconductor wafer introduces variability in temperature measurement that may not be compensated.
What is needed is a method and system for monitoring and accurately controlling temperature in a rapid thermal anneal system.